Fix cbmc-nightly for macOS (#3400)

With macOS, cmake needs a full path to the C++ compiler. (Documented in
CBMC's COMPILING.md.)

.github/workflows/cbmc-latest.yml

@@ -38,14 +38,24 @@ jobs:
           repository: diffblue/cbmc
           path: cbmc
 
-      - name: Build CBMC
+      - name: Build CBMC (Linux)
+        if: ${{ startsWith(matrix.os, 'ubuntu') }}
         working-directory: ./cbmc
         run: |
           cmake -S . -Bbuild -DWITH_JBMC=OFF -Dsat_impl="minisat2;cadical"
           cmake --build build -- -j 4
           # Prepend the bin directory to $PATH
           echo "${GITHUB_WORKSPACE}/cbmc/build/bin" >> $GITHUB_PATH
 
+      - name: Build CBMC (macOS)
+        if: ${{ startsWith(matrix.os, 'macos') }}
+        working-directory: ./cbmc
+        run: |
+          cmake -S . -Bbuild -DWITH_JBMC=OFF -Dsat_impl="minisat2;cadical" -DCMAKE_CXX_COMPILER=$(which clang++)
+          cmake --build build -- -j 4
+          # Prepend the bin directory to $PATH
+          echo "${GITHUB_WORKSPACE}/cbmc/build/bin" >> $GITHUB_PATH
+
       - name: Execute Kani regressions
         working-directory: ./kani
         run: ./scripts/kani-regression.sh